A PLL is a well-known frequency-selective feedback control system which is adapted to generate a signal that can synchronize with a reference input signal and closely track the frequency changes which may be associated with the input signal. PLLs are utilized in numerous applications, including, for example, communication, telemetry and data-recovery systems.
The range of frequencies over which the PLL can maintain synchronization with an input signal is typically defined as the tracking range, or lock range, of the system. This is different from the range of frequencies over which the PLL can first synchronize with the incoming signal, the latter range of frequencies being typically known as the capture range, or acquisition range, of the PLL. The capture range is characteristically always smaller than the tracking range in practical PLL circuits. Generally, the larger the tracking range of the PLL, the worse is the performance of the PLL. Ideally, a narrow tracking range is desirable in order to rapidly capture the input signal and to hold the frequency of the input signal more precisely while in lock. However, a narrow tracking range does not allow the PLL to maintain synchronization with the incoming signal over fluctuations in the frequency of the incoming signal, in addition to variations in operating conditions and/or manufacturing characteristics to which the PLL may be subjected, such as, for example, process, voltage supply and/or temperature (PVT).
A problem exists in the use of PLL circuits for certain applications, such as, for example, frequency multiplication, wherein the PLL output overshoots the targeted operating frequency and comes out of lock. This phenomenon, in which the PLL output frequency increases unrestrained to its maximum possible value, is often referred to as runaway. In a runaway condition, the PLL essentially never re-acquires lock to the reference input signal. Conventional solutions to eliminate or avoid runaway have generally involved monitoring a control voltage generated by a charge pump circuit in the PLL and comparing this voltage to the PLL supply voltage. Under a runaway condition, the charge pump control voltage will exceed the PLL supply voltage by some detectable amount and will cause the comparator to generate a runaway detect signal indicative of the presence of the runaway condition. This runaway detect signal is then used to re-initialize the PLL in an attempt to lock to the reference input signal.
Since a false runaway detection may cause catastrophic data loss, a conservative approach is typically employed to set the failing threshold as high as possible. Unfortunately, over a range of PVT variations to which the PLL may be subjected, when this threshold for detecting a runaway condition is set too high, there is a significant risk that the occurrence of a runaway condition will never be detected.
Accordingly, there exists a need for a PLL circuit which does not suffer from one or more of the above-described problems associated with conventional PLL circuits.